(1) Field of the Invention
The present invention relates to processes for recovering lignin from black liquor within a papermaking operation or a crude lignin waste stream from a biomass enzymatic conversion process. More particularly, the present invention relates to processes for recovering and purifying lignin to produce a low-salt, high-energy-content lignin fuel pellet.
(2) The Prior Art
Lignin, a component of wood, is the second most abundant polymer in the world behind cellulose. Lignin is primarily recovered from the black liquor stream within pulp and paper mills, such as from the kraft pulping process. Black liquor is removed from the host paper mill's recovery system downstream of an efficiently-performing soap separator, since tall oil impurities are deleterious to the operation of the unit operations of the process and the downstream applications, especially the high-value applications other than fuel pellets. Additionally, crude lignin is a byproduct stream from the plethora of technologies using enzymes being developed which convert the cellulose in biomass to ethanol or other products. Those enzymes do not affect lignin which exits those processes in various forms, generally low in solids and with various pH depending on upstream treatments.
With its high energy density and variety of functional groups and structure, lignin holds promise to be an efficient biofuel source or green-chemical precursor. Thus, one use for lignin is to recover lignin as a solid and burn the solid lignin as a fuel, to or use the lignin as a binder for energy pellets. Another use is to provide a process to recover a high-purity low-salt lignin that is used to replace phenol used in resins for composites, to be a natural polymer for making polyurethanes, or to be used in a wide variety of alternative downstream chemical applications.
Currently wood pellets are burned, but the ash content and lower energy density limit their use as a fuel. Lignin pellets have approximately the same energy content as coal, about 12,000 Btu/lb, which is about 50% higher energy per mass of low-moisture wood pellets having about 8,000 Btu/lb. Lignin pellets may be used alone or blended directly with the coal feed with the only additional capital being the separate storage and feeding equipment for the pellets. Also lignin has demonstrated potential as an improved binder for wood or grass pellets, decreasing the dust levels generated in processing of the pellets, improving the water resistance of pellets which is important for outside storage of pellets, and increasing the energy density of the pellets.
Two lignin recovery methods from papermaking black liquor are presently used. The first method, implemented in the 1940s adjacent to a host kraft mill in Charleston S.C., makes powdered lignin containing a high-salt content, which is difficult for power companies to handle. The salt content also creates issues with high ash within power furnaces. Also there is the problem of cooling and diluting the black liquor that is returned to the host paper mills, which creates a high energy penalty in the black liquor recovery operation. The second method, in development since the 1990s, is currently run as a demonstration plant in Sweden. This second method makes low-salt lignin pellets used for fuel, but major issues exist with high wash-water and energy penalty suffered by the host paper mills. The filtrates from the second method have to be returned to the host paper mill to recover the sodium but the black liquor is cooled significantly (from >200° F. to <140° F.) in addition to the wash water, which is added.
Removing a fraction (up to 30%) of the lignin from black liquor allows pulp and paper mills that have reached the maximum throughput of their recovery boilers to increase production by the same fraction of lignin removed. For example, a large paper mill recovering 30% of their lignin from black liquor could produce >50,000 tons of lignin pellets per year. If a papermaking facility makes 50,000 ton/yr of lignin, and that lignin energy value is replaced by burning residual wood, then that lignin is used to displace coal, then the overall green-house gases are reduced by 250,000 ton/yr.
Most pulp and paper mills have the infrastructure to gather residual wood within an economically-effective radius (˜70 miles) of the mill. Many of these mills have reached the limit of their recovery furnaces because of heat-transfer limitations within the furnace. The multiple tubes within the furnace that generate steam on the inside with heat transferred from the burning concentrated black liquor on the outside reach their upper limit of heat flux. Increasing that heat flux risks catastrophic consequences (recovery furnace explosions); thus mills don't exceed that limit. Removing a fraction (≦30%) of the lignin allows the mills to increase their overall production rate of paper by that same fraction.
Many states are implementing renewable energy thresholds on electricity-generating power furnaces, many of which burn coal. However, burning significant fractions of residual wood, as the paper industry does, requires a different design of the furnace, which would have a larger footprint and would require more capital than a coal-burning furnace. A major factor is the lower energy content of residual wood containing significant levels of water (≧40%); wet residual wood has as low as 25% the energy density (Btu/lb) as coal or lignin pellets. To produce energy pellets, the wood has to be dried to moisture contents of 10-20%, but still the energy density of cellulose is still ⅔ that of coal. And residual wood contains significant levels of inorganics, which result in much higher levels of ash within the fuel, which requires either specialized equipment to continuously remove the ash or periodic shut-down to remove the ash. The paper industry historically has build power furnaces capable of burning large fractions of residual wood; the power industry has not. The power industry can add small fractions of residual wood to their furnaces, but a practical upper limit is soon reached. Additionally the power industry and paper industry are frequently at odds, competing for the same supply of residual wood.